Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF T~E INVENTION
This invention relates to a process of and
apparatus for separating air by adiabatic pressure swlng
adsorption. :
In the prior art ad~abatic pressure swing
processes for air separation, the cycle sequence usually
includes a selective adsorption step during which compressed
air is introduced at the adsorbent bed inlet end thereby
forming a nitrogen adsorption front, nitrogen being
selectively adsorbed by most adsorbents as for example, ;;
zeolitic molecular sieves. Oxygen is also coadsorbed but
subs~antially displaced by the more strongly held nitrogen
adsorbate. Oxygen effluent gas is discharged from the
opposite or discharge end of the bed at about the feed air - -
pressure and the ni~rogen adsorption front moves progressive~
ly toward the discharge end. The adsorption step is ;;
terminated when the front is intermedlate the inlet and
discharge ends, and the bed is cocurrently depressurized
with oxygen effluent being released from the discharge
end and the nitrogen adsorption front moving into the
previously unloaded section closer to the discharge end.
The coQurrent depressuriYation gas may in part be
discharged as oxygen product and in part returned to other
adsorbent beds for a variety of purposes, e.g. purging
and pressure equalizatio~ wlth a purged bed for partial
repressurization thereof. _
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Cocurrent depressurization is terminated before the front
reaches the discharge end so that the oxygen purity of the
effluent is nearly that of the gas discharged during the
preceeding adsorption step as for example descrlbed more
completely in Kiyonaga U. S. Patent No. 3,176,444.
The cocurren~ly depressurized bed is usually
further depressurized by releasing waste gas through the
inlet end~ i,e, countercurrently depressurized, until the
bed pressure diminishes to a desired low level for purging.
Then oxygen purga gas is flowed t~rough the bed to desorb
the nitrogen adsorbate and carry same out of the system. The
purged and at least partly cleaned bed is then repressurized
at least partly with oxygen and/or feed air and returned to
the adsorption step, One such process delivering product
oxygen at nearly the feed air pressure is described in Batta
U.S. Patent No. 3,564a816, and requires at least four adsor-
bent beds arranged in parrallel flow relation. Another
process delivering product oxygen at lower, slightly above
atmospheric pressure is described in Batta U.S. Patent No~
3,636,679, and requires at least three beds arranged in
parrell flow relation. Still another process requiring any
two adsorbent beds arran~ed ln parallel ~low relation is
described in McCombs U.S.
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Patent No. 3,738,087. The latter process lncludes an
increasing pressure adsorption step of introducing feed
air to the inlet end of the pa.r~ially repressurized
adsorption bed at pressure higher than the aforementioned
intermediate pressure, selecti.vely adsorbing nitrogen and
simultaneously discharging 02ygen gas, all at relative
rates such that the pressure of the adsorption bed rises
from the intermediate pressure during this step to higher
pressure at the end of such step.
In pilot plant tests relatively high oxygen
recoveries were obtained wi~h both three bed and four bed
systems. For example, in a four bed calcium zeolite A
system in which the bed diameter was six inches and the
feed air was supplied at 70 F and cycled according to the
teachings of the aforementioned Batta U~ S. Patent 3,564,816, ~-
at 90% 2 product puxity the oxygen recovery was 45.5%.
However, in commercial-scale equipment composed of calcium
zeolite A beds 26 inches in diameter, the 2 recoveries
were substantially less than expected9 i.e. 39O4% and 42.3% ~.
at air feed temperature of 50 F and 78 F, respectively,
Also, in a commercial size three bed calcium zeol~te A
system (26 inch--bed diameter~ in which the feed air was
supplied at temperature of 40 F, the 2 recovery was less
than expected. The system stabilized at a product purity
of only 66V/o and with an oxygen recovery of only 26.7%.
Moreover, at feed air tempera~ure of 110 F, the oxygen ~
., .
recovery was only 33~6%o
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An object of this ~nven~ion is to provide an
improved adiabatic pressure swing process for air separation
which permits oxygen recoverles in commercial size equip-
ment which are equivalent to those obtained in ~mall pilot
plant equipment
- Other objects will be apparent from thP ensui~g
disclosure and appended claims
SUMMARY
This invention relates to an adiabatic pressure
swing process of and apparatus for selectively adsorbing
nitrogen from feed air to provide oxygen effluent product.
One of the more important characteristics of
an adsorbent is the selectivity it exhibits for the
components of a multi-component system. Crystalline
zeo1itic molecular sieves of at least four Angstroms pore
size co-adsorb oxygen and nitrogen from air, but selectively
adsorb nitrogen relative to oxygen~ It is known that this
selectivity is temperature sensitive and certain prior art
: suggests that in the crystalline zeolitic molecular sieve- ~
nitrogen-oxygen system, the selectivity for nitrogen improves ;;:
somewhat with increasing temperature~ at least up to room
temper~turel However, Heinze U.S. Patent 3,719,205 teaches
that temperature exerts an opposîte effect by stating that
with
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calcium zeolite A (Molecular Sieve 5A), the separatlon
efficiency of an adsorption oxygen enrichment decreases~ ;~
with incr0asing temperature.
Another important characteristic of adsorption
processes is adsorbent utiliz:ation or capacity for the
adsorbate; it is known that utilization normally decreases
with an increase of adsorption temperature. Karwat U.S.
Patent 3,355,859 teaches that in a pressure swing adsorp-
tion air separation process employing calcium zeolite,
it is necessary to take into consideration that the ;
selectivity of the adsorption material for nitrogen at
lower temperature is lower than at room temperature 3
while the amount of gas adsorbed thereby is much grea~er
than at room temperature. The patentee also states that
a satisfactory oxygen enrichment is achieved if in this
case an adsorption te~perature of -100C and -60~C and
preferably about -70~C. However, Skarstrom U.S. Patent
3,237,377 states that room temperature is preferred for
air separation by pressure swing adsorption using zeolitic ;~
molecular sieve adsorbent.
To resolve the conflicting prior art teachings
regarding the effects of temperature for adiabatic pressure
swing adsorption, air separation studies were conducted on ~ ~
the nitrogen-oxygen-calcium zeolite A system, and Fig. 1 ~ ;
is a graph showing the percent oxygen recovery versus gas
temperature relationship for calcium zeolite A (Molecular
,
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Sieve 5A), for sodium zeollte A (Molecular Sie~e 4A) and
sodium zeolite X (Molecular Sieve 13X). Zeolite A is
described in U. S. Patent 2,882,243; the as-synthesized
sodium form has an apparent pore size of about four
Angstrom units and the calcium exchanged form has an i;
apparent pore size of about Eive Angstrom units. Zeolite
X, another synthetic crystalline zeolitic molecular sieve
is described in U. S. Pa~ent 2,882,244; the as synthesized
sodium zeolite X has an apparent pore size of about ten
Angstrom units. In Fig. 1, the sodium æeolite A curve
is shown by a dashed line, the calcium zeolite A curve
is shown by a solid line and the sodium zeolite X curve
is shown by a dash-dot-dash line. In general, the curves
show that percent oxygen recovery increases with increas-
ing temperature from 0F up to a maximum of about 90F ~`
and thereafter diminishes with further increasing tempera-
ture.
Significantly, the aforementioned four inch
diame~er, four bed system tested at 70~F feed air tempera~
ture yielding 45% oxygen recovery is on the calcium zeolite
A curve, ~ut the commercial size 26 inch system is sub-
stantially below the oyxgen recovery predicted from the
curve and based on the feed air temperatures.
The prior art has taught that in adiabatic
pressure swing processes (which by definition occur with
out loss or gain of heat), the end-to-end bed temperature
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should be uniform. Recognizing that the heat effects
of adsorption and desorption produce cyclic temperature
swings in the bed, each activa particle of adsorbent
absorbs heat and rises in temperature during adsorption.
Durin~ desorption, the particle releases heat and is re-
cooled. At steady state, the quantity of fluid adsorbed by
a particle equals the quantity desorbed; also the quantity
of heat absorbed equals the heat released and the tempera- :
ture rise equals the temperature fall. Therefore, over
each full cycle the next change in temperature is zero and
the adiabatic concept should be applicable to every local
zone of ~he active adsorbent bed. Disregarding these
cyclic temperature swings~ the prior art has assumed that
each adsorbent particle throughout the bed undergoing press-
ure swing adsorption experiences a uniform average temperature
substantially equal to the temperature of the entering feed
air.
Contrary to the prior art teachings of uniform
adsorbent bed temperature during pressure swing air separa-
tion, it has been unexpectedly discovered that these
thermally isolated beds experience a sharply depressed ~;~
temperature zone in the adsorp~ion bed inlet end. As
used herein, "thermally isolated'l means that the beds are
not physically joined to each other for heat ~ransfer ther~
: between, and the 'linlet end" of the zeolitic molecular
sieve adso~bent bed is ~hat portion to which the feed air
is introduced and which adsorbs substantially all
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of any conta~minants contained in the air feed i.e. C02 and
water. The inlet end of the bed includes 30% of the full
bed length, and is measured from the point of air feed
introduction and extending in the direction of air flow
towards the discharge end at which the oxygen product
emerges. In most instances, the inlet and discharge ends
of the adsorbent bed are integral; however, the feed end
may be physically separated from the xemainder of the bed
as long as both portions are directly joined from the
fluid standpoint. This means that each part experiences
the same process step at the same time.
In some instances, the aforementioned depressed
temperature zone in the inlet end has been observed to
experience temperature drops on the order of lOO`F below
the feed air temperature. By way of example, the lowes~
curves in Fig. 2 6how that with a feed air temperature of
O
38 F, a temperature as low as -66 F was measured a distance
of one foot from the inlet end support screen. Fig. 2 also
shows that such temperatura depression exis~s when the
feed air is relatively warm, i.e, 95 F. It is believed
that the inlet end temperature depression is most severe
in those systems which experience an inadvertent heat re-
generative step at such endO Such heat-regenerative step
serves to cyclically rec~ive and storP the chilling efect
of desorption during counter-flow periods of the process
and to cyclically return
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the chilling effeet to the bed during forward flow periods
of the air separation process. When raw air which has not
been pretreated is employed as feed, a water-loaded zone
develops in this region and essentlally no oxygen-nitrogen
separation occurs therein. For purposes of-this invention,
the inlet end temperature depression may be characterized
as producing a temperature difference within the inlet end
(between the point at which feed air is introduced and the
coldest point) ~f a~ least 50 F and with the coldest temper-
ature within the inlet end being no warmer than 35 F. The
temperature depression as hereinbefore described does not
occur in absorbent beds of less than 12 inches effe~tiv2
diameter. As used herein, effective diameter refers to the
minimum cross-sectional dimension of an adsorbent bed. In
smaller beds, there is sufficient heat inleak to the absorb-
ent such that the atmospheric heat moderates the depression
and the process is not truly adiabatic. Also, the inle~ end
temperature depression does not develop unless the feed air
is separated to produc4 at least 6Q% oxygen. With lesser
oxygen-nitrogen separations, the chilling effect of ~esorp-
tion is not sufficient to develop the aforementioned depres-
sion. Although there will always be a degree of depression
irrespective of bed effective diameter or degree of oxygen-
nitrogen separation. In such instances, the depression is
not sufficient to substantially reduce the oxygen re- ~;
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covery and require the practice of this invention.
In the process of this invention, heat is trans-
ferred to the inlet end by metal solid conduction from both
the adsorbent bed inlet and the bed section downstream the
inlet end at rate sufficient to maintain the gas flowing
therethrough at temperature at least 20 F warmer than such
without such metal solid conduction heat transfer, but less
than 110 F. As used herein, "adsorbent bed inlet" refers to
the bed location at which feed air is introduced and which
is proximate to the bed support means, as for example a
metal screen. The absorbent bed inlet is at essentially the
temperature of the feed air. The effect of this inlet end
heating is to move the absorbent temperature to a higher
level along the curves of Fig. 1 and thereby increase the
percent oxygen recovery ~owards the maximum possible value.
The aforementioned temperature comparisons should be based
on measurements at the same point of time in the cycle and
at the same location in the absorbent bed. If there is a
significan~ varia~ion in the temperature difference through
the inlet end, the measurements should be made in the region ~-
of lowest absolute temperature and greatest difference, as
for example in the one foot bed depth region of the Fig.
system. In another process embodiment of this invention,
feed air is introduced at temperature less than 90 F and
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heat is transferred to the inlet end by ~oth an
external heating source and by metal solid conduction,
The heat to be added may be introduced as sensible
heat in warm process streams entering the feed air
inlet end of the adsorbent bed~ In most pressure
swing adsorption air separati.on processes, the feed
air is compressed to superatmospheric pressure and '
the heat of compression is more than adequate to
supply the aforementioned inlet end heating, Adsorbent
bed inlet end heating may also be achieved by intro~
ducing externally generated heat to the air feed, as
for example with a shell-tube heat exchanger employing ~ ~;
steam as the heating medium. Similarly, the external
heat may be introduced to a recycled process stream
from the discharge end of the adsorbent beds, as for
example heating oxygen by an external source prior to
introduction at the feed air inlet end for partial ~
repressurization of a purged bed at low pressure, ~-
More specifically, the broadest process
aspect of this invention relates to an adiabatic
pressure swing process for air separation by
selectively adsorbing at least nitrogen alternately --
in at least two thermally isolated crystalline ;
zeolitic molecular sieve adsorption beds of at least
four Angstroms apparent pore size at ambient tempera-
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9~1'1
~ 4i~34
ture wherein the feed air is introduced to the inlet end of
a first adsorption bed at high pressure and at least 60%
oxygen discharges from the discharge end of the bed. The
first bed is cocurrently depressurized and the cocurren~
depressurization is terminated when the first bed ~s at
a lower pressur~zation is terminated when the first bed is
at lower pressure. Part of such oxygen from the cocurrent
depressurization is returned or recycled for repr2ssurization
of another purged adsorption bed. Waste gas is released
from the first bed inlet end thereby countercurrently
depressurizing same to a lowest pressure and then oxygen gas
is introduced from another adsorption bed discharge end
to the first discharge end as purge gas for desorptio~ of the
nitrogen adsorbate, the adsorbate-containing purged gas
being discharged from the first bed inlet end as waste ~asO
Oxygen gas from the discharge end of an other-then-first
adsorption bed ;s introduced at above said lowest pressure
to the purged first bed or at least partial repressurizat-
ion thereof. In this prior art air separation process the
aforedescribed gas flows are such that a depressed tempera~
ture section is formed in the first bed inlet end wherein
the coldest temperature ~s no warmer than 35 F and the
temperature difference between said colest gas temperature
and a warmest gas temperature within the first bed is at
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least 50 F. Under these conditions the aforedescribed
depressed temperature zone substantially reduces the oxygen
recovery from the process. In this invention, heat is
transferred to the inlet end by metal solid conduction
from both the adsorbent bed inlet and the bed section
downstream the ~nlet end at rate suffici~nt to maintain
the gas flowing there-through at temperature a~ least
20 F warmer than such gas without such metal solid
conduction heat transfer but less than 110 F. In a pre-
ferred embodiment, heat is transferred to the inlet end at
rate of 15 to 150 BTV/hr/sq~ ft. bed cross-sectional area.
In the apparatus aspect of this invention
there are at least two thermally isolated crystalline
zeolitic molecular sieve adsorbent beds of at least four
Angstroms apparent pore size arranged and constructed for
alternate flow of feed air to the inlet end of each
adsorbent bed and discharge of at least 60% oxygen from
the discharge end thereof. The improvement comprises
a multiplicity of metallic elements extending from the
inlet of each adsorbent bed at least one-third the
dis~ance ~oward the discharge ~nd, having a total cross-
sectional area per square foot of adsorbent cross~sec,ional
area A of XK (L8) 2 where L is the adsorbent bed length
(in feet), K is the thermal conductivity of said metallic
elements
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(in BTU/hr. ft.2 F/ft.), and X is the product IUA
for an 8 foot long adsorbent bed with values between
0.5 and 12, said metallic elements being spaced across
the adsorbent corss-sectional area such that the distance
between each adsorbent particl~e and the closest metallic
- element is less than 7 inches. The metallic elements may
for example comprise metal plates or metal rods, and may
extend from the inlet to the discharge end of each absorbent
bed.
The heat source for the bed inlet end depends
on the distance the metallic elements extend towards the
adsorbent bed discharge end. By way of example, i~ the
metallic elements only extend one-third of this distanca,
at least most of the heat is transferred to the bed inlet
end from the adsorbent bed inlet. If the metallic elements
extend the entire distance to the bed discharge end~ a
significant part of the inlet end heat is derived from
bed section downstream the inlet end.
Xt is also contemp~ated that the metallic
elements may have minor discontinuties in the longitudinal
-~ direction, i.e. short gaps.
As will be hereinafter demonstra~ed, this
invention significantly improves the oxygen recovery
from adiabatic pressure swing adsorption air separation
system.
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.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig, l is a graph showing the percent oxygen
recovery versus gas temperature relationship for various
forms of zeolite A and zeolite X.
Fig. 2 is a graph showing the adsorbent bed
temperature ~ersus bed depth for prior art practice and
also a three bed embodiment of the instant invention.
Fig. 3 is a graph showing the cooling rates
of several points within the depressed temperature section
of a zeolite 5A adsorbent bed during air separation startup.
Fig. 4 is a graph showing refrigeration rate
and the maximum temperature difference in a zeolite 5A bed
during air separation.
Fig. S is a schematic longitudinal cross~section
view of a vessel with an adsorbent bed provided with plate-
type metallic elements spaced parallel to each other across
the bed cross-section7 according to the apparatus of this
invention.
Fig. 6 is an end view of the Fig. S apparatus
taken along line 6-6.
Fig. 7 is an isometric view of three plate-type
metallic elements showing the spacing and heat
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transfer relat}onshlps.
Fig. 8 is a graph showing the relationshlp
between maximum temperature difference and KA values for
various spacings of Fig. 5-7 parallel plate-type metallic
elements.
Fig. 9 is a schematic longitudinal cross-
section view of a vessel with an adsorbent bed provided with
rod-type metallîc elements.
Fig. 10 is an end view of the Fig. 9 apparatus
taken along line 10-10.
Fig. 11 is a graph showing the relationship
between maximum temperature difference and RA values for
various spacings of Fig. 9-10 rod-type metallic elements.
Fig. 12 is an end view of an ad~orbent bed
with plate-type metallic elements being radially disposed
in the axial direction. ;
Fig. 13 is a schematic flowsheet of apparatus
according to the invention, suitable for air separation in
each of two adsorbent beds in parallel flow sequence to
produce oxygen.
Fig. 14 is a preferred cycle and time program
for practice with the two bed Fig. 13 apparatus.
Fig. 15 is a schematic flowsheet of apparatus
according to the invention suitable for air separation ~:
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in each of three absorbent beds to produce oxygen with heat
rom feed air compression also transferred to the depressed
temperature section.
Fig. 16 îs a preferred cycle and time program for
practice with the three bed FLg. 15 apparatusO
Fig. 17 is a schematic 1awsheet of apparatus 18
according to the invention suitable for air separation in
each of four absorbent beds to produce oxygen with heat from
embedded electric coils also transferred to the depressed
temperature section.
Fig. 18 is a preferred cycle and time program for
- practice with the four bed Fig. 17 apparatus. `
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ESCRIPTION OF PREFERRED EMBODIMENTS
A series of tests were conducted to determine
the rate of cooldown in the inLet end of a three bed
adsorbent system of the type lllustrated in Batta U.S.
Patent No. 3,6363679 (Fig. 7). The calcium zeolite A beds
were 57-inch diameter circular cross-section, 8 feet long
and each contained 5700 lbs. of 1/16-inch adsorbent pellets.
The thermally isolated beds were equipped with axially
positioned thermocouples and cycled accordlng to the teachings
of the aforementioned Batta patent.
Fig. 3 is a graph showing the observed adsor-
bent bed temperature ( F) at four dîfferent positîons
în the beds during the înitial 48-hour perîod of operation
based on ~eed aîr at about 110 F and 30 psîg. The four
positions are as follows:
Position - Distance from Support Screen (Inle ~;
A3 inches ~ ;
B15 inches
C27 inches
D39 inches
It wîll be apparent from Fig. 3 that the slope
of a curve represents the instantaneous rate of cooldown
for a partîcular bed location during the time period.
Accordingly, for the inîtial perîod
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of operaLion up to about 15 hours the instantaneous
rate of cooldown was highest at location A, nearest
the inlet and progressively diminished to location D
farthest from the inle~. At about 15 hours the loca-
tion A cooldown rate began to decrease and cooldown
was completed after about 36 hours. After about 30
hours the cooldown rate of the next closest location (B)
began to diminish but cooldown was not yet complete ~;
after 48 hours. Locations C and D were continuing to
cooldown at rapid rates after 48 hours.
The Fig. 3 data was also used to determine
the average cooldown rate of the feed end during
various time periods. By way of example, the cooldown
rate was about 50 BTU/hr~ per ft.2 of bed cross-section
6 hours after startup, and declined to about 32 BTU/hr~
per ft 2 of bed cross-section 24 hours after startup,
During the startup period the adsorbent bed temperature
gradient continued to increase and stabilized after 60
hours, when the bed maximum temperature di:Fference
between the lowest temperature in the inlet end and
the warmest temperature in the discharge end was about
100F.
Fig. 4 is a graph of data ~rom this test
depicting the relationship of the cooldown rate and ~;
the maximum temperature difference in the 8-foot
~ 20 ~ "
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1064834
long beds. This relationship is an expression of a tran~
sient condition in the beds; however, it represents an in-
dication of the quantity o~ heat required in th~ inlet end
in order to stabilize the bed temperature differential at
a specified level. By way of example, if it is desired to
maintain a maximum T of 45F it would be necessary to in-
troduce heat to the inlet end at a rate of 50 BTU/hr. per
ft.2 of bed cross-section. It 6hould be noted in this r - -;
spect that if heat is introdu~ed to the inlet end from an
external source as for example in the form of increased feed
air temperature, the temperature level throughout the bed is
increased. As for example illustrated by comparing the low-
est and middle curves in Fig. 2, the overall temperature
level in the beds is increased by raising the feed air tem-
perature from 38F to 95F but the temperature difference
through the beds remains about the same.
This invention utilizes the same temperature dif-
ference which otherwise limits oxygen recovery efficiency,
to warm the inlet end where the undesirable temperature de-
pression normally occurs. From the process standpoint this
is accompLished by transferring heat to the inlet end by ;~
metal solid conduction from both the inlet and the bed ~ `-
section d~wnstream the inlet -`
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end. Both the inlet and the downstread bed section
are sufficien~ly warmer than tlhe depressed temperature
inlet end to substantially warm thP latter to a level
sufficient for improved oxygen recovery if the warming is
at least 20 F above the coldest temperature in tha inlet
end absent the practice of this invention. Since the afore-
mentioned coldest temperature is no warmer ~han 35 F, this
location is warmed to at least 55 F and it will be apparent
r from Fig. 1 that performance will be on a higher portion
of the zeolite adsorbent curve and the percent oxygen re-
covery will be significantly improved. On the other hand ~
the warming should be less than 110 F or performance will ~;
be on a downwardly sloping portioll of the zeolite adsorbent ~ s
curve preferably, sufficient heat is transferred to the
inlet end to maintain the gas flow therethrough at a
c~ ~
maximum temperature between 60 F and 100 F. It should
also be recognized that practice of this invention reduces
but does not eliminate longitudinal ~emperature differences
within the adsorbent beds, so that for air separation
practice Fig. l must be considered in terms of an average
gas temperature throughout the beds. Stated otherwise~
it is a ~ualitative but not quantitivP indication of
percent oxygen recovery achievable by this invention.
:; .
, Figs. 5 and 6 illustrate one apparatus
-~ embodiment of the invention in which vessel 1 having a
circular cross-section is vertically orien~ed and
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provided with air feed port 2 at the bottom and oxygen pro-
duct discharge port 3 at the top. Bed support plate 4 as
for example a metal screen is transversely p4sitioned at
the vessel lower end and retains crystalline zeolitic mo-
lecular sieve adsorbent bed 5. A multiplicity of met~l
pla~es 7 are positioned parallel to each other and ~qually
spaced across the adsorbent bed cross-section. These plates
are contiguously associated with bed support plate 4 at the
absorbent bed inlet so as to provide a solid metal-to-metal
- contact for heat transfer. Plates 7 extend through ~he bed
inlet end 8 where the depressed temperature zone would other~
; wise be formed, preferably into the warmer downstream bed
section 9. Although not illustrated, plates 7 may extend
to the bed discharge end. Since the inlet end 8 may com-
prise up to one-third the bed length~ plates should prefer-
ably extend at least one-hal~ of the bed length to insure
adequate heat transfer from warmer section 9 to inlet end
8. In a preferred embodiment, plates 7 are fonmed of 1/32
to 1/4 inch thick aluminum, equally spaced at 1 1/2 to 8 ~
~ inch intervals across the absorbent bed cross-section. ~;-
`; In Fig. 7, three plates 7a, 7b and 7c are posi-
tioned parallel to each other with 7a - 7b, and 7b - 7c
spaced at distance "2S". Dotted plane a-b represents the
planar mid-point between plates 7a and 7b, and dotted plane
-~ b-c represents the planar midpoint
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between plates 7b and 7c. In practice, the heat is
transferred by solid conduction from product end 9 or the
feed air downstream section of each plate (the upper
shaded section of plate 7c) to the inlet end section of
each plate (the lower shaded section of plate 7c) as
illustrated by the arrows. This heat encounters five
series resistance: . -
(1) The resistance imposed by a radial depth
of bed in the warm section of the adsorbent bed.
(2) The film resistance at the surface of that
pQEtion of a metallic element of the apparatus which is
in the warm section of the adsorbent bed (the upper shaded
section of plate 7c).
(3) The resistance imposed by the metallic
element itself.
(4) The film resistance at the surface of that
portion of the metallic element which is in the cool inlet
end of the adsorbent bed ~l~wer shaded section of plate 7c).
(5~ A res;stance imposed by a radial depth o~
adsorbent bed in the cool inlet end,
Based on recognition of these heat transfer ~ -
resistances, it has been discovered that the apparatus of
this in~ention must be provided with metallic elemen~s having
certain characteristics in terms of :
24- ~ ~
99Ll
8 3 ~
total cross-sectional area per square foot absorbent cross-
sectional ar~a A, adsorbent bed length L (in feet), thermal
conductivity K ~in BTU) hr . f t , 2 F/ f t . ), and X is the pro-
duct KA for an 8 foot lon~ absorbent bed, and spacing rel-
ative to the absorbent particles. In particular, X should
have values between 0.5 and 12, A should be X~ 2 and the
metallic elements are spaced-across the absorbent bed cross-
sectional area such that the distance S be~ween each adsor-
bent particle and the closest metallic element is less than
7 inches.
Fig. 8 is a graph showing the interrelationship of
these variables for the Fig. 5-7 parallel plate-type metal-
.
lic elements at four different S spacings as follows: A = 1inch, B = 4.5 inch~s, C - 6 inches and D = 9 inches, all in
an adsorbent bed length L of 8 feet and when feed gas tem-
. perature was about 90 F. In general, the graph shows that
: smaller maximum gas temperature differences can be maintain- .
~ . . .
~ ed at smaller S spacings. The graph also shows that the
:- maximum gas temperature difference becomes relatively insen-
~ sitive to continued increasP in the product KA above a KA
: value 12.0 BTU/ft./hr. F, ft.2, and that with KA less than
0.5 the thermal resistance imposed by the plate type ele-
- ments pre-dominates and controls the heat transfer rate. ;~
~- Under
": _
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thes~ circumstances the system would be relatively insen-
sitive t~ spacing S and satis:Eactory performance cannot be
achieved with reasonably spaced elements, i.e. spacing of 1
inch or greater. In a preferred embodiment, X is between
1 and 6.
It will be apparent from Fig. 8 that if the S
spacing is greater than about 7 inches, the maximum gas tem-
perature difference in a bed will exceed about 60 F and the
potential improvement in percent oxygen recovery by the
practice of this invention would be very limited. On the
other hand, S spacings of less than 1 inch are ~o be avoid-
ed for mechanical and cost reasons, and spacings of 1 to 3
- inch are preferred as a balance between ease of fabrication
and heat transfer rate.
The Fig. 8 relationship is directly applicable to
beds of 8 foot length. For different lengths the heat tran- -~
~ sfer resistance imposed by the metallic elements of this in-
'`L~ vention is altered by reason of the change in length o~er
- which the heat is transferred. In fact the heat transfer
resistance is directly proportional to the length of the
metallic element. Accordingly, the KA required for a bed
length L is L/8 times the KA required for an 8 foot long
s bed. ~ ~
It should also be understood that adiabatic pres- ~ -
sure swing processes are usually designed for
;, ,
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operation at a specific cycle time and the air feed rate
is adjusted for maximum bed utilization. With bed length
L the feed rate is L/8 times the feed rate employed for an
8 root bed and the KA of the metallic elements must be
altered accordingly. To accommodate both the-change in heat
transfer resistance and the change in quantity of heat trans
fer, ~he KA for a bed of length L is(L) ~ times ~he KA
required for an 8 foot bed,
Use of Fig. 8 is illu~trated by the followîng
example: Assume that it is desired to limit the maximum
temperature difference of gas flowing through a 10 oot long
calcium zeolite A adsorbent bed system for air separation to
40 F so as to achleve about 90% oxygen and the feed temper-
ature is about 90 F. Assume further ~hat alumin~m plates
(K=130) are to be placed in the beds of circular cross-
section and at uniform S spacing of 2.25 inch, i.e. 4.5
inch center-to-center distance between adjacent plates.
~ ,
From Figo 8 and based on a T ordinate of 40 K, a KA value
about 2 may be obtained by extrapolation between the A and
B curves. Since X e~uals 130 A is about 0.0154 for an 8
foot long bed. The value of A for the desired 10
... . . .
9911
106~ 39~
foot bed may now be calculated from the formula X ~ ~ as
followq: (2/l3oxlo/8~2=o~o24 total cross-sec~ional square
feet of aluminum plate per square foot of adsorbent cross-
sectional area Since there are ~2/4.5= 2.67 plates per
square foot of adsorbent cross-sectlonal arsa, the aluminum
plate thickness should be (12) (0.024) /2,6700,108 inch.
Figs. 9 and 10 illus~rate another apparatus
embodiment in which rods comprise the metallic elements 7
arranged in a square pattern, and the Fig. 11 graph shows
the interrelationship between the aforementioned variables
for such rods in a manner comparable to Fig. 8 for the plate-
type metallic elements. Again the adsorbent bed length is
8 feet and the S spacings are as follows: A=l inch, B=4.5
inches, C=6 inches and ~=9 inches. The rods are arranged in
the square pattern of Fig. 10 and the S spacing is one-half
the diagonal distance across the square.
A comparison of Fig. 11 with Fig. 8 reveals tha~
the general relationships are the same and that a spacing
greater than 7 inches should be avoided for the p~eviously
discussed reason. In a preferred embodiment, ~hey are
formed of aluminum of 1/4 to 1 inch diameter and uniformly
~:paced across the ad~:orbent
O
t ~:
-28- ~
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beds at 0.01 to 0.10 square foot total cross-sectional
area per square foot of adsorbent area.
Fig. 12 illustrates another configuration
for plate-type metallic elements, wherein the plates 7
are radially disposed in the axial direction of the
adsorbent beds S. Their outer edges are equally
spaced around the perimeter of the adsorbent beds.
Other suitable forms of metallic elements
include a series of members arranged concentric
with each other around the longitudinal axis of the
bed, in spaced relation with adsorbent material packed ~ -
therebetweeen. They may for example have circular
or square cross-sections, normal to the bed longitudinal -
axis.
Any for example of the aforedescribPd system
for transferring heat to the inle~ end of an adiabatic
pressure swing system for air separation by metal solid
conduct~on can for example be practiced in the two,
three and four bed embodiments of Figs. 13-18. By way
of example Figs. 13-14 illustrate a two bed system of the
increasing pressure adsorption type, i.e. the feed air is
introduced and oxygen product gas is discharged at rela-
tive rates such that the adsorbent bed pressure rises. ~;
~ ' .
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In this type of process, the purged ad-
sorption zone at the lowest pressure level is partially
repressuri~ed to an intermediate pressure by introducing
oxygen gas. The process is characterized by an
increasing pressure adsorption step of introducing
feed air to the inlet end of the partially repres- :
surized adsorption zone at pressure highex than
said intermediate pressure, selectively adsorbing
nitrogen and simultaneously discharging oxygen
from the zone discharge end, with the feed gas in
troduction, the nitrogen adsorption and the oxygen
discharge at relative rates such that the pressure ~:~
of the adsorption zone rises irom the intermediate
pressure during this step to higher pressure at the
end of such step.
Stated otherwise, during the increasing
pressure adsorption step the net molal rate of gas
introduction ~o the adsorption zone is grea~er than ;`
the net molal rate of gas adsorption on the bed, In
this relationship, "net molal rate of gas introduction'l ~.
'~
- 30 - .
. ....... ~ . . : .
9911
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is the rate at which feed air is introduced minus the above-
zero rate at which gas is discharged from the bed, and the
"net molal rate of adsorption" is the rate at gas passed
into the adsorbe~ phase minus the rate at which components
of the feed are displaced or otherwise released from the
adsorbed phase. When the net molal rate of gas introduction
exceeds the net molal rate of gas adsorptlon, the adsorption
pressure will rise. This may be accomplished by restricting
the discharge of oxygen gas relative to the inflow of feed.
The incr~asing pressure adsorption step preferably continues
until the highest pressure level of the process has been
attained and the nitrogen adsorption front has moved from
the adsorption zone inlet end to a position intermediate the
inlet and discharge ends. The lD~ation of the one component
adsorption front is such that a substantial portion of the
zone length downstream of the front is unused, i.e. not yet ;~
significantly loaded with the one component. The adsorption
zone is thereafter cocurrently depressurized for sufficient
duration ~o move the nitrogen adsorption front to the zone
discharge end. During
.
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this period, oxygen is released from the zone and may be
used to repressurize or purge ,another adsorption zone, and/
or be discharged as product. In this manner, the adsorbent
is fully utilized and maximum recovery of the less strongly
adsorbed components is achieved at high purity.
Each step in the cycLe of bed A will now be out-
lined and related to those components of Fig. 13 which are
involved in the cycle changes. Pressures illustrative of
such operation for air separation using calcium zeolite A
adsorbent are included and are related to the following ~ ~ -
terms used herein the identify the terminal pressure in a
relative sense~
Term Illustrative psig.
lowest pressure 1 ~,
lower intermediate pressure 10
equalization pressure 20 ;-
higher intermediate pressure 32 ;~ ;
highest intermediate pressure 35 ;~
highest pressure 40 ~`
Time 0-10: Bed A is being repressurized from the
lowest process pressure (less than 1 psig.) to the equaliz-
ation pressure (20 psig.) and bed B is being pressure eqaul-
ized. Valves 15A and 16A are open and valves 17A and 18A
are closed. Feed air is introduced
, . .
-32-
, .. ~ . . . . .. . . . .. . . . . . .
9~11
~ 6~34
to bed A a~ its inlet end ~rom manifold 11 through valve
15 A and one component-depleted gas from manifold 12 is
simultaneously introduced at the bed ~ discharge end through
valve 16A, The~latter is derlved from bed B undergoing press-
ure equalization through trim valve l9B, valve 16~, and
flows consecutively through valves 16A and tr~m valve l9A
into bed A, Bed B is cocurrently depressurized during this
period and the flow continues for about 10 seconds until
pressures bPtween beds A and B are substantially equalized
at about 20 psig. During this period, the flow o~ equal-
; ization gas is rapid while the flow of ~eed air from the : ~`
compressor is limitad, so that the ma~or portion of the
gas for repressurizing bed A from 0 to 20 psig. is one
component-depleted gas, e.g. 85% for air separation.
.' During this period, another part of the gas released from
' bed B is discharged as product in manifold 12.
:~ Time 10-30: Valve 16A is now closed and flow
of feed a~r only continues to bed A for an additional
twenty seconds to a higher intermediate pressure o~
about 32 psig. Simultaniously the bed B cocurrent
depressurization continues and alL of the nltrogen-
depleted gas released therefrom is discharged as product
~ iD manifold 12. During this period the
: ~ ,
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: : . .. , - ,. .,. . . : .
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bed B pressure diminishes from 20 psig. (equalization)
to 10 psig. (lower intPrmediate). During the bed B pressure
equalization and cocurrent depressurization steps, the
nitrogen adsorption front has moved progressi~ely toward the
bed discharge end, and at this point has reached the discharge
- - end, so that breakthrough is imminent Therefore it can no
longer deliver product purity gas to manifold 12 and valve
.`- 16 B closes. In order for product flow to be uninterrupted, :~
the product gas must be derived from bed A, and in this
process the latter dellvers product during the remainder of
its repressurization,
Time 30-35: Valve l~A again opens and flow of
product proceeds from bed A to manifold 12. This is the
first part of the bed A increasing pressure adsorption etep
and the bed pressure rises from 32 psig,~higher intermediate)
to 35 psig. thighest intermediate). Simultaneously valv~
. 18B opens, waste discharge valve 25 closes and bed B is
~ counter-currently depressurized through its inlet end to
`. less than 1 psig., the lowest pressure of the process,
r Ti~e 35-60: During this remaining part of the ..
bed A ~ncreasing pressure adsorption step wherein the bed
~ pres~ure rises from 35 psig. ~highest intermediate ~to 40
l~ ~ psig. (highest), valves 17B and
'' s '
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991
~ 6 ~ 3 ~
25 are open and part of the nitrogen-depleted gas discharge
from bed A flows through valves 23 and 24 and 17B to purge
bed B.
At the beginning of the bed A 0-10 second
repressurization through both the inlet and discharge ends~
a nitrogen adsorption front is established naar the inlet
end, This front moves progressively toward the discharge
end during the remainder of the lO second perlod and during
the succedding repressurization steps for the first 60
seconds of the cycle. At the end of this period, a pre-
determ;ned length of unloaded bed (unused by nitgrogen)
remains between thenitrogen adsorption front and the
discharge end.
Time-60-70: Valve 15A closes and valve l6B is
opened and bed A now commences pressure equalization with
bed B while continuing to deliver product. Bed A is co- :~
currently depressurized by releasing gas from the discharge
end. The ga~ flows through the unloaded bed length wherein ~
the nitrogen componen~ is adsorbed and the emerging nitrogen- ~:
depleted gas is employed in two parts~ OxygPn product gas
flows through control ~alve 21 in manifold 12 to the
consumer conduit downstream valve 21 at a rate servlng to
hold the consumer conduit at a suitable low pressure sueh
as 3 psig. The remainder and major part of the nitrogen-
depleted gas
-35-
, . . . . . .,, . , . . . , . . ~ ..... .. ... . ..... . .. . . .. .. . . . . . . .
9911
10 ~ ~8 3 ~
flows through valves 16B and l9B to the discharge end of
bed B for partial repressurization thereof. Bed B has
previously been purged of nitrogen adsorbate and is initially
at ~he lowest pressur~ level of the proce~s, This flow of
one component-depleted gas from bed A to bed B continues for
about 10 seconds until the two beds are at substantially the
same equalization pressure of 20 psig. During this step
valve 15B is open and bed B ls also being repressurized
through its inlet end with feed air from manifold ll,
Time 70-90: Valve 16B closes and additional
nitrogen-depleted gas is released from the bed A discharge
end for cocurrent depressurization to about 10 psigo (lower
intermediate), the entire quantity of this ~as from bed A
being discharged as product. Simultaneously only the feed
air flow is continued to the bed B inlet end for further
repressurization thereof from 20 psig, to 32 psig.
Time 90-95: Bed A is now countercurrently :~
depressurized to the lowest process pressure by closing
valves 15A, 16~, opening valve 18A and closing va~ve 25
so the nitrogen desorbate is released through waste manifold
14. Simultaneously valve 16B opens and nitrogen-depleted
gas emerges from the bed B diseharge end for flow through :~
manifold 12 and valve
'~ ' '
3 ' ~ ~
-36- ~ `
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.. ....... . . . . .. ... .. ~ . . ..... .. .. .. .. . . . . . . .. .. . . . .
' ' ' ! ., , ' ' ~ . ; . ` `
.~.. . . .
99
1~6~834
21 as product, This is the first part of the bed B
increasing pressure adsorption step wherein the bed pressure
rises from 32 to 35 psig, dur~ng nitrogen adsorption from
feed air flowlng through the bed,
Time g5-120: Valves 17A and 25 open and part
of the nitrogen-depleted gas emerging from bed B is returned
from manifold 12 through valves 23 and 24 to the bed A
discharge end as purge gas, The latter flows through bed A
countercurrently to the feed gas flow direction and desorbs
the remaining nitrogen adsorbate. The resulting waste gas
is d;scarded through valve 18A and manifold 14. Simultaneously
with the bed A purgin, the bed B increasing pressure reaches
40 psig, the highest -pressure of the process, At this
point valves 17A and 18A are ciosed and purged bed A is
again ready for repressurization in accordance wi~h the `~
foregoing sequence.
As previously stated, the crystalline zeolitic
molecular s~eves useful in the practice of this invention
have an ap~arent pore siæe of at least about four Angstrom-
units, Crystalline zeolites having apparent pore sizes
of at least 4.6 Angstroms are preferred because they permit
more rapid adsorption and desorption of the nit~ogen mole-
cules particularly in the lower temperature
_
~, .
-37-
. .
.. ..
,: ~ . . ....
: -: . , .
~06~334 9911
region, leading to faster operating cycles than attainable
with small pore size zeolites.
The term apparent pore size as used herein may
be defined as the maximum critical dimension of the molecu-
lar species which is adsorbed by the zeolitic m~lecular sieve
in question under normal condi.tions. The apparent pore size
will always be larger than the effective pore diameter,
which may be defined as the free diameter of the appropriate
silicate ring in the æeolite structure.
The term "zeolite," in general, refers to a group
of naturally occurring and synthetic hydrated metal alumi-
nosilicates, many of which are crystalline in structure d '
There are, however, significant differences between the var-
ious synthetic and natural materials in chemical composition
crys~al structure and physical properties such as X-ray
powder diffraction patterns.
r The structure of crystalline æeolitic molecular
sieves may be described as an open three-dimensional frame-
work of SiO4 and A104 tetrahedra. The tetrahedra are cross- -
linked by the sharing of oxygen atoms, so that the ratio of
oxygen atoms to the total of the aluminum and silicon atoms
.
i is equal to two, or 0/(Al+Si)=2. The negative electro-va-
lence of tetrahedra containing aluminum is balanced by the
inclusion within the crystal of -~
r
r
': :
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-38-
. - ~
.... .. ... ~, . . . . .. . .. . ... .. .. .. . .. .. .. . ... . . . . . .. . . . . . . . .... ...
6~L83~ 9911
cations, for example, alkali metal and alkaline earth
metal ions such as sodium, pot:assium, calcium and mag-
nesium ions. One cation may be exchanged for another
by ion-exchange techniques.
The zeolites may be activated by driving off
substantially all of the water of hydration. The space
remaining ln the crystals after activation is available
for adsorption of adsorbate molecules. Any of this space
not occupied by reduced element metal atoms will be avail-
able for adsorption of molecules having a size, shape and
energy which permits entry of the adsorbate molecules
into the pores of the molecular sieves. ;
The zeolites occur as agglomerates of fine
crystals or are synthesized as fine powders and are
preferably tableted or pelletized for large scale adsorp-
tion uses. Pelletizing methods are known which are very
satisfactory because the sorptive character of the zeolite,
both with regard to selectivity and capacity, remains
essentially unchanged.
Among the naturally occurring zeolitic molecular
sieves suitable for use in the present invention include
erionite, calcium-rich ~habazite and fauj site, The
natural materials are adequately described in the chemical
art. Suitable synthetic cxystalline zeolitic molecular
sieves include types A, R, X~ Y, L and T. Zeolites such
-39-
.,,,. ~. . , .................. :. .
.: :
9911
~ 6 ~ 83 ~
as types X, Y, L and chabazite are particularly useful
because of their relatively large pore size~.
Zeolite A is a crystalline zeolitic moleoular
sieve which may be represented by the formula:
1.0~0.2M 2 O:A1203:1.85+0,5SiO2 H20
where M represents a metal3 n :is the valence of M, and y
may have any value up to about 6. The as-synthesized
zeolite A contains primarily sodium ions and is designated
sodium zeolite A. All of the monovalent cation forms of
zeolite A have an apparent pore size of about 4 Angstroms~ :
excepting the potassium form which has a pore size of about
4 Angstroms and consequently iæ unsuitable for use in the
present invention. When at least about 40 percent of the
moncvslent cations sites are satisfied with di- or trivalent
metal cations, zeolite A has an app~rent pore size of about
5 Angstroms.
Zeolite R is described in U.S. Patent No. .--
3,030,181. ~
Zeolite T has an apparent pore size of about ;~:
.
5 Angstroms, and is described in U. S. Patent No. 2,950,952.
Zeolite X has an apparent pore size of about 10 :.
Angstroms, and is described in U.S. Patent No. 2,882,244.
Zeolite Y has an apparent pore siæe of about :~
10 An~stroms, and is described in U.S. Patent ~o. 3,130,007.
~ '
-40-
, ,~,. . . ... .... .............. ...... .. . . . .... ... .. ... ..... .... .. ... ..
~ - . - . . . .
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3~
As previously stated, in one embodiment
of the lnvention heat is t:ransferred to the inlet
end by both an external heating source and also
by metal solid conduction, and means for accomplish~
ing same are illustrated in the three bed Fig. 15
embodiment. The feed air in concuit 11 is compressed
in compressor 20 and heat is introduced to the gas
as heat of compression. ~ormally this heat is
removed in an aftercooler as the adsorbent's
capacity is relatively lower at relatively higher
temperature, Typically this cooling is accomplished
in passageway 50 by heat exchange with water in
passageway 51, but since more than sufficient
heat for this purpose is often introduced as heat
of compression, controlled heat addit~on can be ~;
accomplished conveniently be selecti~ely bypassing
a portion of the compressed air around the aftercooler
through conduit 52 and control valve 53 therein. '~
Although not illustrated, another method for controlling
the net heat of compression added to the feed air
. ~ . .
is by selecti~ely cooling the total compressor
discharge airO This cooling can be accomplished
by regulating the cooling water temperature or
cooling water flow rate.
.
~ 41 ~ ~
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i ~
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991
3 ~
One advantage of transferrin~ heat to the i~let
end by both methods is that smaller and/or fewer metal
elements are needed in the bed. Also, less feed air ,-
heating is needed and this reduces the quantity of
moisture introduced with the feed air.
Figs, 15-16 illustrate a three adsorbent bed
system which may be preferred when the oxygen product
. . .
is to b~ consumed at only slightly above atmospheric
pressure, e.g., as the aeration gas for an activated
sludge waste treatment system. In the latter embodiment, ;
at least the major part of the feed air is introduced
: .:
attendant a rise in adsorbent bed pressure. The bed
pressure rises because the net instantaneous rate of gas~`
introduction ~inflow minus outflow~ exceeds the adsorption
capability of the bed. These embodiments are distinct -~
. ,., ~, ~,
from those wherein at least the major part of the feed air
is introduced during a steady pressure adsorption step,
i.e., wherein the net rate of feed air introduction equals
the adsorption capability of the bed.
:
Referring now to Fig. 15, it shows three adsor~
bent beds A, B, and C connected in parallel flow relation
, between feed air manifold ll, oxygen effluent gas manifold
`~ 12, oxygen purge manifold 13 and waste manifold 14. Auto~
matic valves I5A, 15B and 15C direct feed air flow respec-
tively to fir~t bed A9 second bed B, and third bed C.
~ .
- 42 -
'
~..
~, ..
9~11
- 1~6~834
Automatic valves 16A~ 16B, and 16C respectively direct
effluent oxygen gas from the same beds into manifold 12.
Purge manifold 13 joins one oxy~en efluent gas manifold
12 at the discharge end of the three beds~ and oxygen purge
gas is introdu~ed through automatic valves 17A, 17B~ and
~ 17C to beds A, B~ and C countercurrent to the direction of
feed air flow, Automatic valves 18A, 18B, and 18C join
waste manifold 14 at the inlet end of the corresponding beds
for discharge of countercurrent depressurization gas and
purge gas. Valves l9A, l9B, and l9C at the discharge end
upstream of oxygen effluent valves 16A, 16B, and 16C
respectively are the manual trim type for limiting the flow
of pressure equalization gas.
; Fig. 16 Illustrates one timing sequence suitable
for use with the Fig. 15 system~ employing six distinct
steps each involving commencement and/or termination flows.
Streams flowing into and ou~ of the three bed system are ~
indicated by vertical line flows in the feed manifold 11 and ^~ -
in the oxygen ef1uent gas manifold 12. The feed air manifold
11 connects horizontally with each of the three adsorbent
beds and the latter in turn join horizontally with the oxygen
effluent manifold 12. The repressurization and purge steps
which use a portion of the oxygen effluent are connected
horizontally with the steps, e,g., cocurrent depressurization
and pressure
-43-
9911
~L~6~834
equalization which 9upply the returned oxygen ~as. All
inter-bed flows are identified on the figure.
It will be apparent from Fig. 16 that at any
moment of time one of the adsorbent beds is delivering
product oxygen at progressi~ely diminishing pressure to the
~ product manifold 12 as follows: bed C during 0-40 seconds
bed A during 40-80 seconds and bed B during 80-120 seconds.
Accordingly, product oxygen flow to the consuming means is
continuous.
The utilization within the system of the pressure
equalization and cocurrent depressurization gas is indicated
by horizontal flow lines Each pressure equalization step
is connected horizontally with a repressurization step in
another bed having already been purged, and each concurrent
depressurization step is connected horizontally with a purge
step of a different bed having just been counter-currently
depressurized.
Each step in the cycle of bed A will now be
outlined and related to those components of Fig 15 which
are involved in the cycle changesO Pressures illustrative
of such operation for air separation using calcium zeolite
i~ A adsorbent are included.
.
Time ~-15 seconds: Bed A is being repressurized,
bed B countercurrently depressurized, and bed C pressure
f equalized~ ~alve 15A and 16A are open, and valves 17A and
; 18A are closed. Feed air is introd~ced to bed A at
-44-
. . .
:. ~ , . ,
::;~ ' , , ~,
, .
9911
~ 34
its inlet end from manifold ll, and one component depleted
gas from manifold 12 is simultaneously introduced at the
bed A discharge end, The latte:r is derived from bed C
through trim valve l9C and valv~e 16C, and flows consecu~ively
through valves 16A and trim valve l9A into bed A. Bed C
is cocurrently depressurized during this period and the flow
continues until pressure between beds A and C is substantially
equalized at about 15 psig, During this periodg the flow
of equalization gas is rapld while the flow of feed air from
compressor 20 is limited so that the major portion of the
gas for repressurizing bed A from 0-15 psig is oxygen gas.
During this period, another part of the gas released from
bed C is discharged as product oxygen in maniEold 12.
Time 15-40 seconds: Valve 16A is now closed
and only the flow of feed air continue~ to bed A until ;;-
the terminal pressure of 30 psig is reached. This completes
the repressurization period for bed A, -:;
Time 40-55 seconds: The pressure equalization
step for bed A commences by closing valve 15A and opening .
valves 16A and 16B cocurrently depressurizing the bed by
releasing gas from the discharge end. O~ygen product gas :~
flows through control valve 21 in manifold 12 to the
product conduit do~nstxeam valve 21 at a rate servlng to
hold the product conduit at a suitable low pressure such
as 3 psig . The remainder and maj or part of the
-45-
' '' ' ' ' ' ' ' '' ' "' . , ' '
.,, ~ .
99~1
~ ~ 6 ~ ~ 3 ~
oxygen gas flows through valves 16B and l9B to th~ di.scharge
end of bed B for partial repressurization thereof. Bed B
has previously been purged of nitrogen adsorbate and is
initially at about 0 psig. This flow of product gas from
bed A to bed B continues for about 15 seconds until the two
beds are at substantially the same pressure, e.g,,15 psig.
Time 55-80 seconds: Additional nitrogen-depleted
gas is released from the bed A discharge end for further
cocurrent depressurization thereof, with one part introduced
to the bed C discharge end by closing valve 16B and
opening automatic valve 17C in the purge manifold for purg.ng
nitrogen at slightly above 0 psig. Valves 23 and 24 reduce
the purge gas pxessure to essentially one atmosphere and also
hold the flow rate of purge gas constant. Thisg in turn,
holds the to~al quan~ity of purge gas constan~ since the
purge step is preferably a fixed length of time. The flow
rate is controlled at a steady value by regulating valve 23
which holds the pressure constant between the two valves
23 and 24, The waste gas emerging from bed C inlet end
~lows through automatic valve 18C in waste manifold 14
and.is released ~hrough automatic waste discharge valve ~5.
The last-mentioned valve is a flow-limiting device rather
than the shut-off type. When '~closed1', it introduces a
flow restrict:Lon in$o the waste manifold 14 whi~h reduces
the depressur:ization rate to
-46-
., .. ~ .. .. . . . .. . . . .... .....
9911
i~641!~3~
value below that causlng attrition of the adsorbent particles.
However, for the discharge of purge gas, valve 25 is open
to remove the restriction inasmuch as flow is alxeady limited
by valve systems 23 and 24. Another part of the addi~ional
nitrogen-depleted gas from bed A is discharged as oxygen
product. During this step~ the pressure of bed A and manifold
12 continues to decrease un~il it reaches about 11 psig,
which occurs after an additional 25 seconds ~80 seconds to
cycle). The lowest pressure limit for cocurrent depressuri-
zation, e.g., 4 psig. should be maintained because the pres-
sUxe corresponds to imminent breakthrough of the adsorption
front at the discharge end of the bed, This completes the
production phase for bed A.
Time 80-95 seconds: Bed A now begins its
nitrogen adsorbate rejection (de~orption) phase by closing
valves 16A and 17C, and opening valve 18A. Additional gas
at 4 psig is released from the bed A inlet end ~rom
countercurrent depressurlzation thereof through waste
manifold 14 and discharge valve 25. The latter valve is
~: :
"closed" for this step in order to introduce the aforeo
said restriction and a~oid excessive flow rates from the
bed. This step continues to essen~ially one atmosphere
:.
~, in about 15 seconds.
Time 95-120 seconds: Bed A is purged of remain-
ing nitrogen adsorbate b~ opening valves 17A and 25.
: ~
i .
.~ :
9911
1~Ei483~
Additional nitrogen-depleted gas from the discharge end
of bed B flows through manifold 12 through val~es 23 and
24 and purge manifold 13, then through valve 17A ~o the
bed A discharge end, The nitrogen-containing purge gas
emerging through the bed A inlet end flows through valve
18A and is discharged through waste valve 25, Purging .,
continues for 25 seconds, This completes the cycle and
bed A is in a condition to commenee repressurization with
feed air,
Beds B and C are eonsecutively cycled through
the aforediscussed steps with bed B entering the simul-
taneous feed air-product oxygen repressurization with
the bed A pressure equali~atlon step ~time 40-55seconds~,
Bed C enters the simultaneous feed air-product oxygen
repressurization ~ith the bed A countercurrent depres- -
surization step ~80-95 seconds), The necessary valve
changing for these steps will be reoognized from Figs.6-7
and the foregoing description, A cycle control system
is necessary to initiate and coordinate these valve
changes. The cycle controller may for example receive
a signal from pressure sensing means in feed air conduit
11 downstream compressor 20, ;~
,
Summarizing the aforedescribed three adsorption
bed system which is preferred when oxygen gas product is
to be discharged at low pressure, the first bed is initially
at the lowest pressure and purged of nitrogen adsorbate,
., ,
i -48-
9911
1~ 6 ~3 ~
Feed air and oxygen gas are simultaneously lntroduced
respectively to the first bed lnlet end and discharge
end, ~xygen gas ls simultaneously released from the
dischar~e end of a ~hird bed initially at the highest
superatmospheric pressure with one part discharged as
product and the balance returned to the first bed discharge
end for such simultaneous introduction, the gas flows
continuing until the first and third beds are first higher
pressure equalized. After terminating the oxygen gas
introduction to the discharge end, the feed air introduction
to the first bed inlet end is continued until the bed is
repressurized to the highest superatmospheric pressure.
Oxygen is thereafter released from the repressurized
first bed di~charge end with one part thereof dlscharged
as product and the balance returned to the discharge end
of a partially repressuriæed second bed for simultaneous
introduction during feed air introduction to the second
bed inlet end until the firs~ and second beds are first
higher pressure equalized. The first bed is then cocurrently
depressurized to about 21 psia, The cocurrent depressuri~
zation of the first bed is continued with one part of the
oxygen discharged as product and the balance returned to the
, .
third bed discharge end for purg~ng of nitrogen adsorbate
therefrom. The first bed is thereafter countercurrently
r
~, .
.
9911
1 0 ~ ~3'~
depressurized and oxygen from a cocurrent depressurizing
second bed is then returned to ~he first bed discharge end
for purging thereof. The aforedescribed steps are
- consecutively followed wit~ the second and third beds in
accordance with the flow cycle sequence of Fig. 16,
` EXAMPLE I
, . .
In experiments performed using the above~
described three bed system of Fig. 15 and 16, but without
inl~t end warming of this invention9 the b~ds were 96 inches
long and contained 26 inch inside diameter vessels of circular
cross-section. The adsorbent was 1/16 inch pellets of calcium
zeolite A. The feed was not pretreated to remove C02 and
was water saturated, Each of the vessels contained 1200 lbs.
of adsorbent and the system was fed air at the average rate
of 9100 schf, and at temperature of 100 F. The afore-
mentioned highest euperatmospheric pressure was 45 psiaO
Bed A was equipped with thermocouples located at the
axis of the vessels and at spaced distances from thP air
inlet end to the discharge end. Beds B and C were e~uipped
with an axi~lly positio~ed thermocouple located two feet
into the air inlet of the bed,
-- Product was extracted at the rate of 633 sc~h.
and analyzed for oxygen content. Continued cycling of
:, '
~ ~50~
-- . ,, .. .... .. , . . . . , .. .. ... .. . :.. .. . . . .. ... ..
,. .. , ~,........ .. .. . .
- 9911
1 ~ ~ 4~ 3
the system resulted in a depressed temperature zone in
the inlet end as depicted in the two middle curves of
Fig. 2. In the latter, the curves are used to show the
range of temperatures between the coldest and warmest
section of the beds at the same point in time. Two
curves are plotted for Example I to depict the temperature
variation which is e~perienced at a single point in the
bed. This temperature variation is a measure of the
cyclic temperature effect which is common to adiabatic
pressure swing adsorption processes, and is very small as
compared to the magnitude of the stable end-to-end
bed temperature gradient which developed, ~t is significant
to note that relatively little temperature drop occurs in -
the first few inches adsorbent bed length because this
section is loaded with preferentially adsorbed air im ~
purities (primarily water and CO2) and virtually no -~ -
nitrogen is adsorbed therein. The temperature drops
sharply farther into the first foot of bed length to a
low point about - 21 F at a distance one foot from the
support screen, so that the temperature difference within
D :'~
the inlet end is 120 F. The system stabilized at a
product purity of only 88% oxygen with 29.3% oxygen
recovery.
., ' .
:`
-51 ~
~ ", ., ' ! , ~ ~ .
9911
:~6~3~
EXAMPLE II
.
In another experlme;nt using three bed apparatus
identical to that employed in Example I, the sys~em was
fed air at an average rate of 9030 scfh. and at temperature
of 38 F with 45 psia as the highest superatmospheric pres-
sure an~ again without inlet end warming. Product was
discharged at rate of 770 scfh. Continued cy~ling again
resul~ed in a depressed temperature ~one in the inlet end
a~ depicted in the two lowest curves o Fig. 2, the lowest
temperature being about -67 F at a distance one foot from
the support screen so that the temperature difference within
the inlet end is 105 F. The system stabilized at a product
purity of 66% oxygen witb 36.770 oxygen recovery,
EXAMPLE III ~.
In still another experiment, a three bed appar- :~
atus identical to that used in Examples I and II except that
the adsorbent bed diameter was 24 inches, was equipped with
platetype metallic elements arranged and positioned in the
manner of Figs. 5-7. These plates were. 1/16-inch thick
aluminum spaced 3 inches apart within the adsorbent bed
and extended from support screen at the inlet to the top
of the adsorbent bed at the product end. The total cross-
sectional area o these aluminum plates per square foot of
~dsorbent was
5~ .
.
g911
1~6~334
0.0233 ft. so tha~ ln the aE~rementioned ormula (1) for
A, the X factor is about 2,9.
In this experiment ~ach of the beds contained
950 lbs. of 1/16-inch pellets of calcium zeolite A and the
syst~m was fed unprepurified air at the average rate of
59B0 scfh. The system stabilized at a product purity of
93.4%-oxygen with 38.4% oxygen recovery. The longitudinal
gas temperature of bed A is plotted in Fig. 2 (the upper
` curves)O Although the operating conditions are not
ldentical to Example I they are sufficiently similar to
permit comparison. It will be apparent that without
practice of this invention, the coldest gas temperature
in the inlet would have been about - 25 F and the temper-
ature difference within the bed would have been about 120
With the aluminum elements the coldest gas temperature-in
the bed was about 52 F and the temperature differe~ce
within the inlet end was only 44 F. The oxygen product
purity was increased from 88% oxygen to 93.4% oxygen and
the oxygen recovery increased about 31%, a very substantial
imprsvement.
- In the four bed embodiment o~ Fig~. 17 and
18, external heat is introduced to the inlet end 8 o the
adsorbent beds by electric hea~er or Eluid conducting tube
coils 55 embedded therein,
;~
-53-
" . . . : . ,. .: : .
9911
~ 6 ~3 ~
The preferred location for embedded heat exchange
mechanisms ls within the initiial 15% of bed length,
The externally supplied heat can be added by any appropriate
exchange mechanism as for example a shell-tube unit employ-
ing steam as the heating medium.
When the product oxygen is needed at substantially
the same pressure as the eed air, a four bed system as
for example described in Batta U.S. Patent 3,564,816 is
particularly suitable as hereinafter described and
illustrated in Figs. 17 and 18. Although the selective
adsorption will be only described in terms of removing
nitrogen from the feed air stream to produce oxygenl it
will be understood that atmospheric impurities, primarily
water and C02 b~t also trace amounts of light hydrocarbons,
are also selectively adsorbed in preference o oxygen by
crystalline zeolitic molecular sieves of at least four
Angstromspore size. These impurities are desorbed from
the adsorbent bed during the low pressure purging, along
with the nitrogen
~i .
t
!. -54
.. ... .
. . .
.. ~ . ,
- Fig. 4 shows four aclsorbent beds, A, B, C, and
D connected in parallel flow relation between feed air
manifold 10 and unadsorbed procluct oxygen manifold ll.
Automatic valves lA, lB, lC ancl lD direct feed air flow
respectively to first bed A, second bed B, third bed C
and fourth bed Do Automatic valves 2A, 2B92C and 2D,
respectively, direct product oxygen from the same beds
into product manifold 11.
The adsorbed nitrogen rejected by counter-current
depressurization and purge through waste maniold 12 at the ~
inlet end of the beds. Adsorbers A and B are joined a~ ~;
their inlet ends by conduit 13 having automatic valves 3C `~
and 3D therein. Adsorbers C and D are joined at their i~let
ends by conduit 14 having automatic ~alves 3C and 3D therein.
First stage equalization conduit lS is provided
joining the discharge ends of adsorbers A and B; similarly
first stage equalization conduit 16 is provided joining
the discharge ends of adsorbers C and D. To provide first
stage pressure equalization, automatic valves 4AB and 4CD
are located in conduits 15 and 16, respectively. Valves
17 and 18 in series with equalization valves 4AB and 4CD, ~;
respectivel~, are man~al preset throttling devices which `-
prevent excessively high flow rates from occurring and which ;
allow adjustment and balancing of equalization rates ~-
between the adsorption bed pairs AB and DC.
::
:' ;
9911
lOG4~34
Automatic valves 5A, SB, 5C and 5D are provided
at the discharge ends of the beds, two of which open
- together to pass cocurrent depressurization gas ~rom one
adsorbent bed for use as purge gas in another bed Manual
valves 19 and ~0 in the purge cross-over conduits 21 and
~ 22 respectively serve the same purpose as explained pre-
viously for valves 17 and 18 in the first stage pressure
equalization circuit. The purge cross-over conduits 21
and 22 (piped in parallel flow relation) also contain back
pressure regulators 23 and 24 oriented in opposite flow
directions so as to control flow in either direction between
either bed A or B and bed C or D. The back pressure regulators
i 23 and 24 are set to maintain a min~mum pressure,e.g., 50 p.s.i.
~ in the bed undergoing cocurrent depressurization. When this
r pressure is reached the cocurrent depressurization and purge
steps terminate. This arrangement prevents extension of
cocurrent depressurization to excessiv~ly low pressure with
resultant breakthrough of the one component's adsorption
front.
As previously indicated, valves 17, 18, 19 and
20 are flow rate limiting devices which prevent bed damage
due to e~cessive~P and fluid velocity. A similar precaution
may be followed during countercurrent depressuri~ation, by
means of preset throttle valve ,
t
.~ . . , ................. . ~.,
'
. ~ ' , . .
334 9~11
which acts as a bypass around main waste valve 26 in
waste conduit 12. During countercurrent depressuriza-
tion the automatic main waste valve 26 is closed which
forces the gas to follow the bypass route through valve
25. During the following lowest pressure purge step,
valve 26 opens to minimize flow reslstance in the waæte
condu~t 12.
Repressurization conduit 27 having constant
flow control valve 28 therein joins product manifold 11
for introduction of unadsorbed product efflùent from the
adsorber (on the absorption step) to a different adsorber
having been partiallY repressurized to lower intermediate
pres~ure. Conduit 27 in turn joins product return con
duit 29 s~mmunicating with repressurization valves 6A-6D
joining the product conduits to adsorbers A-D respectively. ;
Cross-over conduits 43 and 44 carry the released gas from
the second lower pressure equalization steps of beds A
and B to beds C and D, and from the latter to the former,
resp ctively. At the inlet end four additional ~equencing
valves 7A, 7B, 7C and 7D are provided in conduits 45 and
. .: . .
46 joining beds AB and CD, respectively.
Second stage pre~sure equalization conduit 40
sommunicates at opposite ends with the bed A discharge
- end through valve 5A, bed B discharge end through valve
5B, bed C discharge end through valve 5C and bed D dis-
`: :
.. '
.:
2:
--5 7--
.~ .
33~ 9911
charge end through valve 5C and bed D discharge end
~hrough 5D. Flow conduit 41 is controlled by valves
41 and 42.
The adsorption step is terminated when the
ni trogen adsorption front is entirely within the bed.
This point may be determined in a manner well known to
those skilled in the art, using the f~ed conditions,
and the adsorbent's capacity and dynamic characteristics.
: Also the first pressure equalization step and the co- ;
current depressurization step are ~topped when the adsorp- :
tion front is till entirely within the bed and before
breakthrough. This permits removal of the nitrogen from
the void space gas by adsorptlon within the bed discharge
end, so that the emerging equalization gas and the purge
~ gas have virtually the same purity as the product gas. `
'',r,' If the cocurrent depressurization step is conducted
before the second equalization step then all void g s
recovery steps must be completed while the adsorption front
ic still entirely within the bed receiving feed air. If the
. second equalization step is carried out after the eoeur-
rent depressurization step, the former may conti~ue past
the breakthrough point as the emerging gas is used for
~ feed end repressurization. Breakthrough may for e~ample
f~_ be identified by monitoring the nitrogen concentration
in the discharge gas, and detecting the moment at which
this concentration appreciably increases. The purge
,, ~
~' ~ r
I ~
.;
_ 5 ~ _ .
~6483~ 9911
step is most efficien~ly performed by removing only the
adsorbables deposited in the preceding step. That is, the
bed is not completely cleaned of all nitrogen by the purge
fluid, but the latter's counter-current flow insures that
the adsorption front is pushed back towards the inlet end.
This insures a clean product during even the initial portion ~-
of the succeeding adsorption s~ep.
The use of the Fig. 17 system to practice the
four-bed embodiment will be more easily understood by
reference to the Fig. lB cycle and time program. There are
six distinct steps each involving commencement and/or
termination of flows. Streams flowing into and out of the ~ -
four-bed system are indicated by vertical lines connecting
the feed manifold 10, the unadsorbed product oxygen effluent ; ;-
manifold 11 and the desorbate waste nitrogen manifold 12.
The feed manifold 10 connects vertically with each of the
four adsorption steps and the latter in turn join vertically
with the product manifold 11. Th~ countercurrent depress-
urization and purge steps, during which the adsorbed nitro-
gen is discharged from the beds, are connected vertically
with the desorbate waste manifold 12. The repressuri~ation
steps which use a portion of the unadsorbed product oxygen
effluent are connected vertically with the product manifold
11. All gas flows associated with the f~ur beds are identi-
fied on the figure.
,-
_~9_
~ 3 4 99~1
At lea~t four adsorbent beds are needed tomatch, timewise, those steps in wh~ch cocurrent depres-
surization streams become available with those steps
which can utilize these streams. Otherwise large holdup
tanks would be required. It will be apparent froM Fig. 18
that at any moment of time, one of the adsorbent beds is
on its adsorption step deli~erlng product at substantially
constant pressure to the product maniold 11. At th~ same
moment the other three beds are being cocurrently depres-
surized, or first or second stage pressure equalized,
cleaned of the adsorbed component and/or repressurized
respectively for the succeeding adsorption step~ One of
the beds i8 always receiving product gas for r~pressuriza-
tion so that the consumption of product for this purpose
iS continuous rather than intermittent.
In Fig. 18 the utilization within the system
of the pressure equalization and cocurrent depressuriza-
tion gas is indicated ~y horizontal flow lines. Each
first ~I) pressure equalization step is connected horizon-
tally with a repressurization step in another bed ha~ing
already been partially repressur;zed, and each second (II)
pressure equalization step is connected horizontally with
a repressurization step of a different bed having just
been purged. Each cocurrent depressurization step is
connected horizontally with a purge step in a different
bed.
, ~
, .
. ~
, ~
-60-
~ .
~ ,
.j~, . . ~
10~;~834 9911
Each step in the cycle of bed A will now be
outlined and related to those components of Fig. 17 which
are involved in the cycle changes. Pressures illustrative
of such operation are included.
Time 0-60 seconds: Bed A is on adsorption at
40 psig. Valves lA and 2A are open, and valves 3A, 4AB,
5A and 6A are closed.
Time 60-78 seconds: At the end of the adsorption
step, valves lA and 2A close, and valve 4AB opens to commence
first-stage pressure equalization between beds A and second
bed B. At this moment, all other valves associated with bed
B are closed except valve 6B (valves lB, 2B, 3B, 7B and 5).
Valve 17 limits the flow rate of equali~ation gas to avoid
bed fluidization, the direction being countercurrent to
feed gas flo~ in bed B. ;
Time 78-102 seconds: When pressures in beds A
and B have equalized at a higher intermediate level of about
26 psig, valve 4AB closes and valves 5A, 19 and 5C open
allowing purge gas to flow from bed A into third b2d C
countercurrent to feed gas flow. At this moment 7 all other
valves associated with bed C except valve 3C are closed
(valves 2C, lC, 4CD and 6C)o Valve 23 throttles ~nd limits
the flow of purge gas so that bed C remains at substantially
one atmosphere pressure.
Time 102-120 seconds: At the end of the purge
step ior thLrd bed C, iirst bed A will have depreiturized
.:
.
-61-
i
~ ,
9gll
)6~L834
to about 16 psig. At this point, valve 3C closes 50 ~hat
the continued flow of gas from bed a into bed C is bottled
up. The continued flow of gas cannot be carried by the purge
crossover (conduit 21, valves 23 and 19) because the
regulator valve 23 is set to terminate the purge flow when
the pressure ~n bed A has dropped to the predetermined lower
limit for the withdrawal of purge gas (e.g,~ 16 psig).
Therefore, the co~tinued gas flow for countercurrent
pressurization of bed C is shunted through conduit 43 by
opening valve 7C and closing valve 23. Beds A and C equal- :
ize at a lower intermediate pressure o~ about 8 psig~
Time 120-138 seconds: First bed A is now
countercurrently depressurized to assentially one atmos~
phere pressure as the lowest pressure of the process by
closing valve 5A and opening val.ve 3A~ Valve 26 in the
was~e conduit 12 also closes forcing the blowdown gas
through flow-restrictive device.25.
Time 1~8-1~2 seconds: Purge gas for the
first bed A is obtained from concurrent depressurization ~f :~
fourth bed D which is between its two pressure-equalization
. .
s~eps~ Valves 5A, 20 and 5D open to permit this flow counter-
current to the previously flowing feed gas. ~t this time,
aIl valves associated with bed D other than valve SD are
closed, Valve 24 throttles and limits the flow of purge
gas so tha~ bed A remains at substantially one atmosphere.
Valve 26 in the waste conduit 12 is also reopened so as to
minimiza flow resistance to the low-
;
.. -62-
,~
' ' ' ,,, ' '
10~4834 9911
pressure purge gas.
Time 162-180 seconds: Bed A is now cleaned and
ready to be repressurized cocurxently. The initial phase
of repressurization is accomplished by continued introduction
of void space gas rom fourth bed D. Valves 3A and 20 close
and valve 7A opens to permit flow of gas from bed D to bed A.
This partial cocurrent repressuri2ation of first bed A
continues until it is pressure equalized with fourth bed D
at lower intermediate pressure, e.g., about 8 psig. This
is also the second or lower pressure equalization stage of
bed ~.
Time 180-198 seconds: The next phase of bed A --
repressurization is accomplished by higher pressure equali-
zation with second bed B which has just completed its ad- `
sorption step and is initially at fulI feed pressure. Valves
5A and 7A close, and valve 4AB opens to admit void space gas
discharged cocurrently from bed B. Valve 17 limi~s the
flow to prevent bed fluidization. This further counter- - `
current repressurization of first bed A continues until it
is pressure equalized with second bed B at higher inter~
mediate pressure, e.g., about 26 p9ig . This is the first
or higher pressure equalization stage of bed B.
T~e 198-240 seconds: The final phase of bed A .
repressurization to substantially feed pressure is accom-
lished with product gas discharged from third bed C through
manifold 11.
~' :
,
i~
~ .
-63-
~6~l93~
Although preferred embodiments o the inven-
tion have been described in deta11, it is contemplated
that modiflcations of the proosss may be made and that
some features may be employed without others~ all with-
in the spirit and scope of the invention.
- 64-
, ,., . , . - , .
